1. Field
The following description relates to a Pulse Width Modulation (PWM) controlling circuit and a Light Emitting Diode (LED) driver circuit having the same. For example, the following description relates to a PWM controlling circuit to generate a PWM signal to control boosting of an LED array according to connection state of a plurality of LED arrays, and an LED driver circuit using the same.
2. Description of Related Art
A Liquid Crystal Display (LCD) is not as thick and weighs less than other display devices. In addition, the LCD requires low driving voltage and power consumption. However, the LCD requires light to operate. As such, since the LCD is a non-light-emitting device that cannot produce light it needs to operate as a display device, a separate backlight is required.
A Cold Cathode Fluorescent Lamp (CCFL) and a plurality of Light Emitting Diodes (LEDs) are used as backlight for the LCD. However, the CCFL can pollute the environment with mercury. In addition, the CCFL exhibits slow response time and low color reproduction, and is not suitable for use in a panel of the LCD that is thin and light.
On the other hand, LEDs are eco-friendly without using harmful substances and allow impulse driving. In addition, LEDs exhibit good color reproduction, arbitrarily change brightness and color temperature by adjusting the light intensity of red, green, and blue LEDs, and are suitable for use in a panel of the LCD that is thin and light. Therefore, LEDs are mostly implemented as the backlight light source for LCD panels.
Meanwhile, when the LCD backlight using the LEDs connects LED arrays including a plurality of LEDs in parallel, a driver circuit supplies constant current to each LED array. Further, a dimming circuit arbitrarily adjusts the brightness and the color temperature to compensate for the temperature.
To maintain uniform brightness and color in the backlight, the driver circuit boosts the driving voltage applied to the LED array. In this case, when the LEDs forming the LED array are open, the voltage of a particular node of the LED array becomes grounded (GND) in the LED Integrated Circuit (IC). Accordingly, the driver circuit performs a continuous boosting operation. At this time, without an overvoltage protection device for the driving voltage applied to the LED array, the boosting of the driving voltage destroys the LED IC.
To prevent this problem, a conventional overvoltage protection technique detects the voltage of a particular node where the driving voltage applied to the LED arrays is divided by a resistor array, and aborts the boosting when the voltage of the particular node exceeds a reference threshold. However, since the driving voltage applied to the LED array is changed according to the change of the LED inch, the conventional technique should separately adjust the resistance value of the resistor array every time the LED inch is changed. As a result, development and test process costs increase.
To supply the constant current to the LED array, it is necessary to fully boost the driving voltage applied to the LED array. However, to increase the driving voltage of the LED array in the conventional technique, the driving voltage is not boosted when a dimming signal is off and is boosted only when the dimming signal is on. Disadvantageously, it takes a long time to fully increase the driving voltage of the LED array.